This invention relates to pressure sensors for sensing fluid pressure.
Batch fabricated differential pressure sensors formed of brittle materials are known, for example, from U.S. Pat. No. 4,833,920 to Knecht et al. (PCT publication number WO 88/00335) which is hereby incorporated by reference. Typically, a sensing diaphragm is formed in a silicon layer that is bonded to glass reference plates which have metal capacitor electrodes deposited on them. The assembly of diaphragm layer and reference layers is then clamped between support cylinders which reduce the tendency of the reference plates to bow when high line pressure is applied. The sensor is connected to electrical circuitry which senses the pressure sensitive capacitances between the capacitor plates and the diaphragm and provides an output representative of the pressure.
Environmental conditions other than a pressure to be sensed can deflect the diaphragm or reference plates undesirably. This undesirable deflection limits the accurate range of such sensors. Mismatches between the temperature coefficients of expansion of the materials used for the diaphragm, reference plates or clamping arrangement can deflect the diaphragm undesirably, or cause stress in the sensor which leads to cracking. Bulging of the reference plates due to line pressure can adversely affect the measurement of a differential pressure. The strength of the bonding and sealing between the diaphragm and the support layer, or between support layer and tubing connections to an isolator can limit the upper range of pressure which can safely be applied to the sensor, or can lead to non-repeatable or hysteresis type errors with changes in temperature, pressures or both.
There is a desire to extend operating ranges of differential and line pressures, temperature, or both, however, limitations in the materials used in the sensor, geometric arrangement and bonding materials continue to limit performance. There is thus a need to provide a pressure sensor which combines materials bonded in a geometry that provides desired accuracy over a selected range of line pressure, differential pressure and temperature.